The present invention relates to an electrophotographic photoreceptor excellent in charge-transporting property and mechanical properties, and a process cartridge and an electrophotographic apparatus using the same.
In recent years, an electrophotographic technology has played a central part in an image output field of copiers, printers and facsimiles because of advantages that high printing qualities can be obtained at high speed. As a structural material of an electrophotographic photoreceptor which is the heart of the electrophotographic technology, inorganic photoconductive materials such as selenium, a selenium-tellurium alloy and a selenium-arsenic alloy have been widely used from the beginning. Meanwhile, the study and the development of an electrophotographic photoreceptor using organic photoconductive materials which are superior to these inorganic photoreceptors in costs, productivity and disposability have been increasingly conducted, and they have currently exceeded the inorganic photoreceptors. Especially, selection of the materials can be more freely conducted by introduction of functional separation designing that charge generation and charge transport being basic steps of photoconduction are conducted by separate materials, and the remarkable improvement of performance has been achieved on the basis of the diversity provided by organic materials. At present, a functional separation laminated organic photoreceptor in which a thin-film charge-generating layer having a charge-generating function and a thick-film charge transport layer having a charge transport function, a chargeability and mechanical strengths are laminated on an electroconductive substrate has been mainly used as an electrophotographic photoreceptor.
Most of the organic charge-transporting materials are hole-transporting materials such as triphenylamine derivatives and hydrazone derivatives, and electron-transporting materials having performance of practical level are rare. Because of this restriction of materials and the structure made of the electroconductive support, the charge-generating layer and the charge transport layer, the organic photoreceptor has generally such a restriction of an driving polarity that it is driven only by negative charging.
With respect to the driving polarity, there are desirable polarities according to a sub-system and a total system used. In order to use the organic photoreceptors more widely and efficiently, the development of negative charging-type, positive charging-type and further bipolar charging-type photoreceptors has been in demand.
For example, when a wire discharge-type charging unit such as an inexpensive scorotron is used as a charging sub-system, there is a problem that harmful gases such as ozone and nitrogen oxides are generated in the discharge. It is known that amounts of the harmful gases generated are smaller by one figure in the positive charging than in the negative charging with the same charging unit. Thus, a positive charging-type photoreceptor has been demanded in a less costly and/or small-sized electrophotographic apparatus in which a filter for removing harmful gases cannot be mounted.
On the other hand, with respect to the requirement of the high image quality, it is advisable to conduct discharge area development using a negative charging-type photoreceptor in using the existing developer. At this time, to cope with the removal of harmful gases, it is required to use a filter of activated carbon or to use a charging unit of a contact charging system in which amounts of harmful gases are reduced.
Further, when the discharge area development system is employed to conduct transfer electrostatically, the charging polarity in transfer is reverse to the charging polarity in latent image formation. Since the charging in transfer is conducted in theory through a transfer material such as paper or a film for primary transfer, a photoreceptor is not directly charged. However, the photoreceptor is actually charged with polarity which is reverse to that in latent image formation between the transfer material and the next transfer material. In this case, the charging with reverse polarity in the transfer cannot be optically extinguished in the photoreceptor which is actuated only with monopolarity. Accordingly, a problem sometimes occurs that in some ability of the charging unit, the history thereof appears in the next copy or print. This problem is basically solved by using a bipolar charging-type photoreceptor having a hole-transporting property and an electron-transporting property.
Meanwhile, the functional separation laminated organic photoreceptor has the following defects owing to the laminated structure, and the development of a photoreceptor capable of conquering these defects has been in demand.
With respect to the first defect owing to the laminated structure, there are problems that since two or more layers have to be formed in comparison with the single-layer photoreceptor, the productivity is decreased and the costs are increased.
With respect to the second defect owing to the laminated structure, when a dip coating method is used which is a film-forming method generally employed because of high mass productivity, care must be taken not to attack a lower layer in coating an upper layer and a resistance to an upper layer coating solvent is required for a lower layer material, and there are problems accompanied by the same.
That is, a charge-generating layer which is generally a lower layer is made of a pigment as a charge-generating material and a binder resin having a film-forming property. A material which is not attacked by a coating solvent of a charge transport layer as an upper layer has to be selected as the binder resin. Generally, the charge-transporting material has a relatively low polarity. A solvent having a relatively low polarity, such as toluene, chlorobenzene, tetrahydrofuran, dioxane or dichloromethane is used as the coating solvent. Accordingly, as the binder resin of the charge-generating layer, a thermoplastic resin having a high polarity, such as an acetal-modified vinyl alcohol resin or a vinyl chloride-vinyl acetate-maleic anhydride copolymer resin having a low solubility in the solvents is employed.
It has been; however, known that a charge-transporting property is decreased in resins having a high polarity. Further, the resins having the high polarity are sometimes problematic in that they have a high moisture absorption and cause change in photoelectric characteristics accompanied by high environmental change. Further, since these thermoplastic resins having the high polarity are not completely insoluble, somewhat roughening is unavoidable in coating an upper layer, which sometimes results in the change in photoelectric characteristics and non-uniformity of the image. Thus, the improvement has been demanded in the application to an electrophotographic apparatus requiring a high image quality of full color.
In order to avoid this problem, the use of a thermosetting resin as a binder resin of a charge-generating layer has been proposed. However, there are a pot life problem that a curing reaction proceeds in a coating solution over the course of time to cause thickening and gelation and a problem that an uncured site remains in film formation and it has an adverse effect on photoelectric characteristics. Thus, the very problem remains substantially unresolved. Further, the charge-generating material such as a pigment cannot completely be insoluble in the coating solvent of the charge transport layer. Consequently, the contamination of a coating bath of the charge transport layer proceeds owing to elution of the charge-generating material with the increase in number of coatings of the photoreceptor, which induces the restriction of the pot life of the coating solution.
With respect to the third defect caused by the laminated structure, when a coherent light of a laser is used as an exposure light source, there arises a problem that a defective image of a stump pattern generally called interference fringe is formed by specular reflection in the interface between the charge-generating layer and the charge transport layer.
In order to avoid this problem, a method in which the interface is rendered uneven and a substrate surface is roughened to scatter light and a method in which a film having a rough surface is formed between a substrate and a photoreceptive layer have been employed. However, these methods themselves involve the increase in costs and require a difficult step of production in which a thin-film charge-generating layer has to be uniformly formed on the rough surface.
Incidentally, as a product to dispel the problems caused by the laminated structure, an electrophotographic photoreceptor of a single-layer structure (monolayer-type photoreceptor) has been long known. However, satisfactory functional separation designing has not been applied to the ordinary monolayer-type photoreceptor, so that it does not have characteristics capable of withstanding practical use. That is, in the ordinary monolayer-type photoreceptor, an insulating resin simply contains a charge-generating material and a charge-transporting material. Owing to the restriction of the transport charge polarity, the concentration of the charge-generating material has to be increased so as to allow optical absorption only in the vicinity of the surface. As a result, there are problems that dark attenuation is increased and photoelectric characteristics are greatly changed due to repetitive use (in the laminated photoreceptor, this problem is controlled by decreasing the film thickness of the charge-generating layer).
Regarding this problem, a monolayer-type photoreceptor in which an insulating resin contains three components, a charge-generating material, a hole-transporting material and an electron-transporting material has been lately proposed, and the remarkable improvement of performance has been attained (Denshishashin Gakkaishi, vol. 30, pp. 274-281, 1991). That is, the restriction of the transport charge polarity is removed by adding not only one of the hole-transporting material and the electron-transporting material as in the ordinary photoreceptor but both of these materials. Even though charge is generated inside the photoreceptive layer of the electrophotographic photoreceptor, both charges are transported to opposite poles by the respective charge-transported materials. Consequently, the amount of the charge-generating material c;m extremely be decreased, thereby solving the problem.
However, a resin generally used for a charge transport layer, such as a polycarbonate, is employed as the insulating resin. Thus, it has been problematic in that a dispersibility of the charge-generating material in a coating solution for forming a photoreceptive layer is poor.
As stated above, the charge-generating layer of the functional separation laminated photoreceptor is made of the charge-generating material and the high-polarity resin having a high-polarity group such as a hydroxyl group or a carboxyl group. In the coating solution for forming the charge-generating layer, the high-polarity group of the high-polarity resin is adsorbed on fine particles of the charge-generating material to inhibit agglomeration of the fine particles and provide a stable dispersed state.
When the high-polarity resin is used as the insulating resin of the functional separation monolayer photoreceptor following this thought, the dispersion stability of the coating solution for forming the photoreceptive layer is improved, but a chargeability, a repetitive stability and an environmental stability of the photoreceptor are notably worsened. That is, as stated above, the high-polarity resin is substantially problematic in a charge-transporting property, a moisture absorption and an insulating property. In the functional separation laminated-type photoreceptor, the adverse effect is only minimized by decreasing the thickness of the charge-generating layer containing the high-polarity resin. Thus, it cannot be applied to a monolayer-type photoreceptor in which a photoreceptive layer has a certain thickness.
Further, in order to satisfactorily provide both a hole-transporting ability and an electron-transporting ability in this system in which the hole-transporting material and the electron-transporting material are dispersed in the insulating resin, there is a need to increase a total concentration of these low-molecular charge-transporting materials relative to the insulating resin having mechanical strengths. Thus, there has been a problem that mechanical strengths as a whole film are decreased.
For improving this problem, there is a method in which a polymeric compound is used as one of the charge-transporting materials to provide a film-forming property and an insulating resin is dispensed with. Inventions using specific hole-transporting polymers are disclosed in Japanese Patent Laid-Open No. 256,050/1991, 249,706/1993 and 150,495/1993. However, the hole-transporting polymers used in these inventions do not have themselves the satisfactory mechanical strengths, and further improvement is also required regarding a discharge product resistance and a compatibility with an electron-transporting material. Further, a dispersion stability of the charge-generating material is also problematic in the coating solution for forming the photoreceptive layer.
Moreover, the electrophotographic photoreceptor involves the following problems.
In the electrophotographic photoreceptor, an antioxidant is sometimes added to the photoreceptive layer for protecting the photoreceptor from oxidative gases generated from a charging unit. Further, for improving mechanical strengths of the photoreceptive layer, a crosslinking agent is sometimes added to the coating solution for forming the photoreceptive layer. However, the antioxidant and the crosslinking agent sometimes have an adverse effect on the charge-transporting property. Thus, it has been difficult to add these materials in amounts capable of providing satisfactory effects without having an adverse effect.
Accordingly, the invention has been made under these circumstances in the related art, and it intends to provide an electrophotographic photoreceptor capable of conquering the problems. The invention provides an electrophotographic photoreceptor having a high performance and a long life which photoreceptor can provide various properties required for a photoreceptor, as occasion demands, by forming a bipolar charge transport layer excellent in hole- and electron-transporting properties, mechanical strengths, a discharge product resistance and a coating dispersion stability as one layer.
The invention further aims to provide a process cartridge and an electrophotographic apparatus having a high image quality and a high durability by using the electrophotographic photoreceptor having the excellent properties.
The present inventors have assiduously conducted investigations on a bipolar charge transport material and a pigment dispersion technology, and have consequently found that the problems can be solved by using a block copolymer or a graft copolymer. Moreover, during the investigations, it has been found that the block copolymer or the graft copolymer generally takes a micro-phase separation structure so that when different functions are imparted to respective structural blocks, they can independently exist in mutually different phases and the respective functions are exhibited to the maximum extents without having adverse effects on the mutual functions, and that the invention can therefore be developed widely in designing the materials requiring plural functions. These findings have led to the completion of the invention.
That is, the invention is an electrophotographic photoreceptor having a bipolar charge transport layer with a hole-transporting property and an electron-transporting property, the bipolar charge transport layer containing a block copolymer or a graft copolymer.
The different functions required for the photoreceptor can be provided together at high levels by imparting the different functions according to the purposes to the respective structural blocks of the block copolymer or the graft copolymer (hereinafter sometimes simply referred to as xe2x80x9cblock copolymer or the likexe2x80x9d) incorporated in the bipolar charge transport layer. That is, the different functions are imparted to the respective structural blocks, and the respective structural blocks are subjected to micro-phase separation. Consequently, the materials and the groups having the respective functions are concentrated in the phases to avoid dilution by mixing. Further, the interference of effects of the materials and the groups having the different functions can be inhibited because the respective phases undergo micro-phase separation.
For example, when a block copolymer or the like contains a block having at least one group selected from high-polarity groups such as a hydroxyl group, a carboxyl group and an alkoxysilyl group and having a pigment dispersion stability and a block free of the high-polarity groups, a high pigment dispersibility can be secured by minimizing the adverse effect on a charge-transporting property, a moisture absorption and a chargeability. Accordingly, when this is applied to a monolayer type, a functional separation monolayer-type photoreceptor having excellent properties is realized.
In the invention, it is preferable that the block copolymer or the like has a charge transport active block and a charge transport inactive block. It is more preferable that the charge transport active block is a hole-transporting block containing at least one structure represented by formula (I) as a recurring unit. 
wherein Ar1 and Ar2, independently from each other, represent a substituted or unsubstituted aryl group, X1 represents a divalent hydrocarbon group or a hetero-atom-containing hydrocarbon group having an aromatic ring structure, X2 and X3, independently from each other, represent a substituted or unsubstituted arylene group, L represents a divalent hydrocarbon group or a hetero-atom-containing hydrocarbon group, and m is an integer of 0 or 1.
In the invention, it is advisable that the bipolar charge transport layer contains a charge-generating material. The charge-generating material is preferably an organic pigment, and the content thereof is preferably between 0.1 and 10% by weight.
The bipolar charge transport layer can be used in various organic electronic devices, other than the electrophotographic photoreceptor provided by the invention, such as an organic electroluminescence device, an organic photorefractive device, an organic photosensor and an organic solar cell.
The electrophotographic photoreceptor of the invention is preferably used in a process cartridge which is able to attach to and detach from an electrophotographic apparatus including at least the electrophotographic photoreceptor.
Further, the electrophotographic photoreceptor or the process cartridge of the invention is preferably used in the general electrophotographic apparatus.